Heavy trucks and trailers have been used for many years to transport goods from one location to another. When a truck delivers a trailer to its destination, the truck may be separated from the trailer so that the truck can be coupled to another trailer for transporting the goods contained therein to another location.
A trailer coupled to a truck receives electrical power from the truck to operate various electrical circuits in the trailer. Such electrical circuits include trailer lamps, stop/brake lamps, and turn signal lamps. The electrical circuits on a trailer are typically wired to a connector that mates with a connector wired through a truck/trailer electrical interface to the electrical systems of the truck. A standard connector in this regard for the heavy truck and trailer industry is an SAE J560 connector.
Present-day wiring for a truck/trailer electrical interface includes high-power relays, fuses, circuit breakers, and numerous electrical interconnects. A number of problems exist with the present method of interfacing a truck's electrical systems to a trailer. One problem is that the fuses and circuit breakers are typically not designed to provide protection for specific circuits. Moreover, in some circumstances, the fuses and circuit breakers trip at current levels far in excess of the nominal current rating of the wiring. Circuit breakers can also fail in ways that lead to a wiring failure. If these devices fail in a shorted mode, they no longer protect the truck wiring from excess current levels. Furthermore, present-day wiring of a truck/trailer electrical interface provides no protection from after-market suppliers connecting additional circuitry to one or more trailer circuits and drawing an excessive amount of power from the truck, leading to electrical overloading of the truck/trailer wiring.
In addition to providing power to trailer lamps, stop/brake lamps, and turn signal lamps, a truck also typically provides power to an auxiliary power circuit in the trailer. At the present time, heavy electrical loads connected to a trailer's auxiliary power circuit may encounter a low-voltage situation in which the available voltage in the auxiliary power circuit is insufficient to operate the connected loads. The low-voltage situation results from the high amount of circuit resistance in the wiring of a present-day truck/trailer electrical interface. As a trailer draws an increasing amount of current through the truck/trailer electrical interface, the voltage drop across the electrical interface also increases.
For example, a typical truck/trailer electrical interface in the heavy truck industry may include a fuse having a cold resistance of 0.002 ohms, 25 feet of #10 GXL wiring having a resistance of 0.115 ohms, a 30-amp circuit breaker having a cold resistance of 0.21 ohms and relay contacts having a resistance of 0.2 ohms, resulting in a total interface resistance of 0.527 ohms. As shown in Table 1 below, when a trailer circuit draws 5 amps of current through the electrical interface, the voltage drop across the interface is 2.64 volts, leaving 11.16 volts available to the trailer circuit (given that the battery voltage of a truck battery is nominally 13.8 volts when the battery is in a charged state). If the trailer circuit were to draw 9 amps through the electrical interface, the voltage drop across the interface would be 4.74 volts, leaving 9.06 volts available to the trailer circuit. Should the trailer circuit attempt to draw a full-rated load of 30 amps, by Ohms law the voltage drop across the interface should be 15.81 volts, which is impossible because it exceeds the total voltage of the battery. Even an available voltage level of 9.06 volts (at 9 amps) is unacceptable for certain trailer systems that require a minimum voltage of 9.5 volts (e.g., a trailer ABS system that per TMC RP144 requires a minimum 9.5 volts at the ABS unit).
TABLE 1 ______________________________________ Drop Across Voltage Battery Present Available Current Voltage Interface to Trailer ______________________________________ 5 amps 13.8 volts 2.64 volts 11.16 volts 9 amps 13.8 volts 4.74 volts 9.06 volts 30 amps 13.8 volts 15.81 volts -- ______________________________________
While trailers of today may not require high amounts of current such as 30 amps, many trailers require about 5 amps for the turn signal lamps, 10 amps for the trailer clearance and marker lamps, and 12 amps for the auxiliary power circuit. In any event, the voltage available to trailer circuits through present interface designs has been shown to be marginal. See, for example, studies performed for trailer ABS systems as reported by the Department of Transportation in DOT HS 808 076 (October 1993). In the future, trailers are anticipated to include circuitry for monitoring loads, weights, and performance that will require additional electrical power. The existing interface wiring methods are not able to meet the increased need for electrical power.
Furthermore, a conventional truck/trailer electrical interface does not provide means for data communication between the truck and a trailer. Federal laws are soon to require that trailer ABS system faults be reported to the driver. Additionally, cargo, weight, tire and brake wear system providers are also looking for ways to communicate status information from a trailer to a driver in the truck. The present invention provides solutions for these needs as well as addresses other shortcomings in the prior art.